Electrochemical testing systems have become increasingly important in the detection and determination of extremely small concentrations of various electrochemically active species in dilute solution, e.g., in the detection and determination of trace impurities in water, or the detection and determination of trace compounds in biological samples. The invention is particularly useful when electrochemical sensors are used in combination with chromatography and will be described in connection with such utility although other uses are contemplated. Several such systems are described in my earlier U.S. Pat. Nos. 4,404,065, 4,497,199 and 4,511,659, the disclosures of which are incorporated herein by reference.
In such systems, the counter and reference electrodes typically comprise thin solid metallic terminal wires or the like, and the working ("test") electrodes typically comprise porous conductive materials such as fritted carbon or fritted graphite which have a very high surface area to volume ratio to permit effective charge transfer to essentially all of a very small quantity of a particular species of liquid passing adjacent the surface of the working electrode.
The present invention is particularly directed to an improved electrochemical sensor, more specifically for an electrochemical sensor for use with liquid chromatography which provides time separated species in the eluant fluid. A particular embodiment of the invention is specifically directed to a technique for modifying the diffusion rate of transitory species with respect to the counter and/or the reference electrodes. Another embodiment of the invention is specifically directed to a technique for modifying the selectivity of porous working electrodes to make them specific for certain groups of compounds. In such systems, the eluant, containing the species to be subject to charge transfer preferably flows through the porous electrode.